1. Field of the Invention
The present invention relates to a gear train for a 6-speed automatic transmission used in vehicles. More particularly, the invention relates to a gear train for a 6-speed automatic transmission utilizing a compound planetary gear set consisting of planetary gear sets combined radially, the automatic transmission thereby having a short length and a simple construction.
2. Description of the Related Art
Generally, automatic transmission systems for vehicles are provided with a transmission control unit which automatically controls shift ratios according to changes in a running condition of the vehicle.
The typical transmission control unit controls a plurality of friction elements provided in a gear train to either operative or inoperative states to select one of the three essential elements of the planetary gear set (a sun gear, a ring gear and a planetary carrier) to be an input element, one to be a reaction element, and one to be an output element, thereby controlling an output number of revolutions.
There are different kinds of compound planetary gear systems used in automatic transmission systems including a Ravigneaux-type, a Simpson type, and a 2 Simpson-type compound planetary gear system.
Also, the above compound planetary gear systems, so as to allow for use as a shifting means for vehicles, have a structure in which planetary carriers of a first simple planetary gear set are connected to a planetary carrier of a second simple planetary gear set, a structure where the planetary and ring gear are connected through the sun gear, or a structure in which the planetary carriers of the first simple planetary gear set and a ring gear of the second planetary gear set are directly connected to each other, namely, a structure wherein two planetary gear sets are connected in a direction of an axle.
However, in the typical automatic transmission, as a length of the system in the direction of the axle is increased, mounting becomes difficult, especially when applying the automatic transmission of a front-wheel-drive vehicle in which the engine is mounted in a transverse direction, and, as a result, much time and effort must go into shortening the automatic transmission in the designing process.
Further, when the output power of a large output power engine is not sufficiently utilized, the typical automatic transmission has a poor fuel consumption ratio, power efficiency, and driving performance.
Therefore, to efficiently use the engine power, increasing the shift ratio has been proposed.
However, in the typical automatic transmission, as only one input element should be selected, at least two planetary gear sets must be provided, thereby increasing costs for manufacturing and complicating the construction.